The invention relates to an illuminating device having an optical integrator, especially for a system for projecting or displaying images. It also relates to such a system.
In the projection (or display) of images, especially of television type, it is known practice to use a liquid-crystal optical valve, the transparency of each image-forming element of which is controlled as a function of the luminance of each point of the image to be formed.
The luminous energy for the projection is delivered by an illuminating device behind the valve and an optical system in front of the valve projects the image onto a screen. It is also possible to display the valve either directly or by way of an optical magnifying system.
In order for the projected or displayed image to correctly restore the image formed in the valve, it is necessary for the illuminating of the valve to be uniform. This problem is not easily solved, in particular, because the illumination originates from a source of limited extent.
One solution consists in using an optical integrator. An example of a known projector having an integrator is depicted in FIG. 1.
In this example, the source 10 is associated with a reflector 12 and the pencils of light reflected by this reflector are collected by a first array 14 of small lenses 141, 142 . . . .
The shape (contour) and the proportions of each lens 14i correspond, preferably, to the shape and to the proportions of the optical valve 16. For example, if the valve is rectangular with proportions of 4:3 or 16:9, the lenses will exhibit the same proportions. With each of these lenses 14i there is associated another small lens 18i of a second array 18. This lens 18i is arranged in such a way that it forms the image of the lens 14i over the whole extent of the valve 16. Thus, even if the various pencils of light originating from the reflector 12 have different energies, this heterogeneity does not result in a heterogeneity of illumination on the valve 16, since the energy of each pencil is distributed over the whole surface of the valve. Furthermore, if the shape and the proportions of each lens 14i correspond to the shape and to the proportions of the valve 16, losses of light are minimized.
The reflector 12 is, for example, elliptical and the source 10 is arranged at the first focus and the pencils received by the lenses 14i are slightly convergent.
A projection objective 20 is provided downstream of the valve 16.
This projection system is of fairly high cost, especially since its various optical components are of large dimensions.
The invention makes it possible to reduce the dimensions of the optical components.
It starts from the finding that the aperture of the projection objective depends on the angle a at which the centre 0 of the valve sees the second array 18 of lenses and that it is therefore beneficial to decrease this angle and, as a consequence, to decrease the extent of the integrator.
Starting from this finding, the inventors have observed that the surface of the array 18 is not used in an optimal manner. This observation appears in FIG. 1a where it may be seen that the array 18, of circular general shape, is illuminated at localized sites 221, 222, etc., that is to say sites which are separated from one another, the sum of the areas of the illuminated zones being substantially less than the area of the disk formed by the array 18.
The illuminating device according to the invention is characterized in that it comprises a means for focusing the light pencils from the reflector onto the integrator.
Thus, each lens of the first array is illuminated at a considerable angle; as a result of this the associated lens of the second array, which lens is intended to image the first lens on the valve, also receives a light beam of considerable angle, this making it possible to increase, preferably maximize, the area of the illuminated part of the second array of lenses. Furthermore, it is possible to minimize the dimensions of the optical integrator, thereby minimizing its cost. Decreasing dimensions of the arrays of the integrator leads to the desired decrease in the angle at which the valve sees the integrator and hence a decrease in the aperture of the projection objective.
Decreasing the aperture of the projection objective makes it possible to increase the contrast since it is known that the use of light rays which are steeply inclined with respect to the normal to the plane of a liquid-crystal valve leads to a decrease in the contrast.
To focus the beam from the reflector onto the integrator, there is provided either a focusing lens or an appropriately shaped reflector, for example an elliptically shaped reflector in which the source is arranged at the first focus and the integrator at the second focus.
The position of the integrator at a point of convergence of the beam makes it possible to use this integrator in a device for projecting or displaying colour images employing a single valve in which each image point is formed from several (in general three) image elements, each of these elements being assigned to a specified colour (red, green and blue, for example), the optical illuminating system being such that a light beam of a specified colour reaches only the image element assigned to this colour. The coloured beams are created from a source of white light and from means of angular separation according to colour. The position of the integrator is such that it does not modify the relative orientation of the rays and thus makes it possible to retain the angular separation of the colours.
In one embodiment, each beam of a specified colour illuminates a part of the surface of the integrator which is assigned thereto. Stated otherwise, the surface of the integrator is divided into several distinct parts and each part is used for a single colour. As a variant, provision is made for juxtaposed integrators, the number of integrators being equal to the number of colours.
In embodiments in which maximum illumination on the valve is desired, a collecting lens is employed between the integrator and the valve so as to superimpose the various beams reaching the screen.
If no collecting lens is used, it is possible to maximize the uniformity of illumination and hence the contrast of the projected or displayed image.